1. Field of the Invention
The present invention relates to a rubber bushing which is employed in a suspension apparatus of an automobile, for instance.
2. Description of the Prior Art
A rubber bushing has been employed in an automobile suspension apparatus such as a semi-trailing arm type automobile suspension apparatus or the like. When a rubber bushing is employed in an automobile suspension apparatus, such a rubber bushing will be hereinafter referred to as a member mount. The rubber bushing is required to exhibit a low spring constant so as to improve the riding comfort. In particular, the rubber bushing is required to have a low spring constant in the longitudinal direction of an automobile in order to suppress the generation of the harshness, and the rubber bushing is required to have low spring constants in the longitudinal and lateral directions of an automobile in order to suppress the generation of the booming noises. However, the rubber bushing is required to have a high spring constant at least in the vertical direction of an automobile in order to improve the driving stability. Thus, the spring constant requirement for the driving stability contradicts the spring constant requirement for the riding comfort. Accordingly, the spring constants of the rubber bushing are selected appropriately in order to improve the driving stability without impairing the riding comfort.
FIGS. 9 and 10 illustrate a conventional rubber bushing. FIG. 9 is a cross sectional view of the conventional rubber bushing taken in a direction perpendicular to an axis thereof, and FIG. 10 is a cross sectional view of the conventional rubber bushing taken along an axis thereof. In FIGS. 9 and 10, an inner cylindrical metal fitting is designated at 101, and an outer cylindrical metal fitting is designated at 102. The inner cylindrical metal fitting 101 and the outer cylindrical metal fitting 102 is made integral by way of a rubber elastic member 104 which has hollows 103 and 103 therein. The hollows 103 and 103 are placed at positions which are symmetrical with each other with respect to the axis of the inner cylindrical metal fitting 101. Further, intermediate metal fittings 105 and 105 are buried in the rubber elastic member 104 in a direction which is perpendicular to the axis of the rubber bushing and which crosses the symmetrical disposing direction of the hollows 103 and 103, and the intermediate metal fittings 105 and 105 are curved in a circumferential direction.
The thus constructed rubber bushing is disposed in an automobile suspension apparatus in the following manner. Namely, the cylindrical axial direction specified by the arrows F and F' of FIG. 10 agrees with a vertical direction of an automobile body, the disposing directions of the hollows 103 and 103 specified by the arrows of U and U' of FIG. 9 agree with a longitudinal direction of an automobile body, and the disposing directions of the intermediate metal fittings 105 and 105 specified by the arrows L and L' of FIG. 9 agree with a lateral direction of an automobile body. Thus, the spring characteristic of the rubber bushing in the vertical direction of an automobile body is made soft by stresses which act in the shearing direction of the rubber elastic member 104, and the spring characteristic of the rubber bushing in the lateral direction of an automobile body is made stiff by the intermediate metal fittings 105 and 105. Consequently, the ratio of the spring characteristic in the axial direction with respect to the spring characteristic in the direction perpendicular to the axial direction is enlarged.
In addition, when the thickness of the rubber elastic member 104 is varied, the spring characteristic of the rubber elastic member 104 in the shearing direction is not varied greatly. For instance, when the thickness of a rubber elastic member is varied by 10%, the spring characteristic of the rubber elastic member in the shearing direction is varied by about 1%. On the other hand, when the thickness of the rubber elastic member 104 is varied, the spring characteristic of the rubber elastic member 104 in the compression direction is varied greatly. for instance, when the thickness of a rubber elastic member is varied by 2% to 3%, the spring characteristic of the rubber elastic member in the compression direction is varied by about 20% to 30%. The inventors of the present invention have thought of obviating the intermediate cylinders 105 and 105 by utilizing the spring characteristics of the rubber elastic member 104 while maintaining the above-described functions of the conventional rubber bushing. Namely, as illustrated in FIG. 11, a ring 106 can be formed integrally with the inner cylindrical metal fitting 101 at an intermediate portion thereof, or, as illustrated in FIG. 12, the inner cylindrical metal fitting 101 can be formed into a barrel shape at an intermediate portion thereof. However, it is extremely hard to form the inner cylindrical metal fitting 101 which has the ring 106 formed integrally therewith. Accordingly, the construction of the modified inner cylindrical metal fitting 101 would result in a problem which complicates the forming process, and in another problem which increases the overall weight of the rubber bushing. Further, it is also extremely hard to form the inner cylindrical metal fitting 101 into a barrel shape at an intermediate portion thereof.
Moreover, irregularities can be formed on the ring 106 illustrated in FIG. 11 in order to make the ring 106 a non-circular, thereby enhancing the effect of the provision of the ring 106. Namely, as the hollows 103 and 103 and the intermediate metal fittings 105 and 105 are disposed symmetrically with each other and as they appear every 90.degree., the ring 106 can be processed to have irregularities having different shapes which appear every 90.degree.. However, such a construction would further complicate the forming process.
As having been described so far, it is hard and complicated to make the inner cylindrical metal fitting 101 of the conventional rubber bushing into a non-circular shape in order to enlarge the ratio of the spring characteristic in the axial direction with respect to the spring characteristic in the direction perpendicular to the axial direction. Further, it is also hard and complicated to integrally form a non-circular portion on the inner cylindrical metal fitting 101 of the conventional rubber bushing in order to enlarge the same.